This invention pertains generally to apparatus for effecting two-to-four wire conversion in communication systems. More particularly, the invention is directed to a subscriber line interface circuit (SLIC) providing electronic hybrid circuitry for interconnecting a two-wire subscriber loop to a four-wire transmission medium while simultaneously providing d.c. battery feed to the subscriber loop.
One typical prior SLIC is taught in U.S. Pat. No. 4,041,252 issued to Earl T. Cowden and assigned to the assignee of the present invention. That SLIC and variations of it have been widely and successfully utilized in the System 1210 electronic switching system manufactured and sold by International Telephone and Telegraph Corporation.
The Cowden SLIC includes provisions for battery boost, or range extension, applications such that the total d.c. battery potential injected into the line is increased. A combined battery and voice injection circuit is coupled to either side of the subscriber line by a resistance. For a line requiring a 900 ohm a.c. termination impedance, a 450 ohm resistor is connected to each side of the line.